Conventionally known cylindrical protectors to be fitted to the outer circumferential walls of hoses including those used with vehicle engines are made of thermoplastic resins such as vinyl chloride resins.
This kind of protector has the following problem. If a protector having a larger inner diameter is used, so as to make the insertion of a hose thereinto easy, the space between the hose and protector makes it difficult to unite the hose and protector together. On the other hand, if a protector having a small inner diameter is used, to reduce the space between the hose and protector, the insertion of the hose into the protector is difficult.
To eliminate the above problem, a heat-shrinkable tube is used which is larger in diameter than a hose inserted thereinto, and upon heating, the tube shrinks to form a tight seal on the hose.
Heat-shrinkable materials used for producing such heat-shrinkable tubes and other heat-shrinkable products include resins such as vinyl chloride resins, polypropylene resins, polyamide resins, fluoroplastics, and the like and rubbers such as ethylene-propylene-diene copolymer rubbers (EPDM), chloroprene rubbers (CR), acrylonitrile-butadiene copolymer rubbers (NBR), and the like.
Known heat-shrinkable tubes have the following problems. Heat-shrinkable tubes made of a resin are too hard when in the enlarged-diameter state and after heat shrinkage. Conversely, heat-shrinkable tubes made of rubber cannot retain their expanded shape, and are too soft when in the enlarged-diameter state and after heat shrinkage.
Heat-shrinkable tubes made of a resin or rubber generally have low thermal conductivities and thus cannot be heated at a high rate. Low thermal conductivity makes temperature control difficult and the article to be covered with the heat-shrinkable material is heated to an undesirably high temperature.